(a) Field of the Invention
The present invention relates to a method of performing synchronization in a distributed wireless communication system and a terminal supporting the same.
(b) Description of the Related Art
Most wireless communication systems use a synchronization method because there are many advantages if terminals are synchronized with each other in a wireless communication system. In a synchronization method, a time domain is divided into repetition intervals that are periodically repeated. Each of the repetition intervals is divided into slots, that is, smaller intervals, and some operations of a terminal are performed in respective specific slots. One of the advantages of the synchronization method is to efficiently support sleep mode for reducing power consumption. That is, if terminals within a network are aware of the structures of repetition intervals and slots, they operate in transmission or reception mode only in some slots within an interval or slots, but maintain an inactive state, thereby being capable of reducing power consumption.
There are some methods in which terminals belonging to the same network are synchronized with each other or maintain synchronization. As in a cellular system, there is a central control type of synchronization method, which is a method in which a specific system or terminal provides a signal, that is, a basis for synchronization (hereinafter a “synchronization signal”), and the remaining terminals are synchronized using the provided synchronization signal. Furthermore, there is a distribution type of synchronization method in which a specific system or terminal does not send a synchronization signal, but terminals within a network themselves participate in the transmission of a synchronization signal.
The distribution type of synchronization method includes a physical layer distribution type of synchronization method. In the physical layer distribution type of synchronization method, when terminals within a network send or receive a specially designed synchronization signal, they are synchronized using the time when the synchronization signal is received. The synchronization signal used in the physical layer distribution type synchronization method includes a pulse signal, a chirp signal, and a modulated Zadoff-Chu (ZC) signal. The physical layer distribution type synchronization method is advantageous in that the time when each of synchronization signals is received can be estimated if two overlapping synchronization signals have orthogonal characteristics although the two synchronization signals are overlapped and received. If a radio channel for a synchronization signal is used by another device, however, inter-signal interference may occur when the synchronization signal is transmitted. Accordingly, the physical layer distribution type synchronization method is disadvantageous in that it effectively operates only when a radio channel for a synchronization signal is always secured.
The distribution type of synchronization method further includes a method of sending additional information related to synchronization when a synchronization signal is transmitted. For example, time offset information may also be transmitted in a synchronization signal. A terminal that has received the synchronization signal performs network synchronization using the time when the synchronization signal is received and a time offset value. Such a method is useful if a radio channel is shared with another communication system in a non-license band or if a collision with a synchronization signal transmitted by another terminal is to be avoided (i.e., if it is impossible to always send a synchronization signal at a desired time). For example, this method is useful when a synchronization signal is transmitted using a contention-based media access control method, such as carrier sensing multiple access with collision avoidance (CSMA/CA).
If networks operating according to the distribution type of synchronization method are spatially separated from one another, they may meet if a radio channel between the networks is changed. In this case, what two networks “meet” means the state in which some or all of terminals belongs to one network are capable of receiving the transmission signals of some or all of terminals that belong to the other network. For example, a case where spatially separated networks meet may occur if the networks move and thus the distance between terminals belonging to the networks becomes close or if an object that hinders communication between two networks disappears.
If two networks that are spatially separated meet, they can be out of synchronization. That is, when two networks are out synchronization, the repetition intervals are out of synchronization. If two networks that are not synchronized meet as described above, they may generate mutual interference, leading to a problem in network operations.